1. Field of Invention
The present invention relates to structures of piezoelectric devices, such as piezoelectric resonators or piezoelectric oscillators, having a piezoelectric resonator element received in a package. The present invention also relates to manufacturing methods therefor.
2. Description of Related Art
FIGS. 8(a) and 8(b) show an example of a conventional piezoelectric resonator, and for the convenience of understanding, a lid body thereof is omitted in the figures. FIG. 8(a) is a plan view showing the interior of the piezoelectric resonator by removing the lid body, and FIG. 8(b) is a schematic cross-sectional view showing the interior taken along plane VIIIb—VIIIb (shown in FIG. 8(a)) by removing the lid body.
In FIGS. 8(a) and 8(b), a piezoelectric resonator 10 has a package base 12 in the form of a box in which a space portion 13 is formed for accommodating a piezoelectric resonator element 11 in the form of a plate. One end 11a of the piezoelectric resonator element 11 is fixed on gold-plated electrodes 14 and 14, which are two mounting electrodes disposed on a step 19 formed in the space portion 13, by bonding with silicone-based conductive adhesives 15 and 15 provided therebetween, and the other end 11b is a free end.
In this structure, the piezoelectric resonator element 11 is formed of, for example, quartz crystal, and on the surface thereof, electrodes (not shown) are formed which apply a driving voltage to the quartz crystal for performing a predetermined oscillation. As a material, which is not shown in FIGS. 8(a) and 8(b), for sealing the package base 12, a metal, such as Koval, or a ceramic, such as alumina, is used.
As a material for the package base 12, a ceramic, such as alumina is used, and in the case shown in FIGS. 8(a) and 8(b), on a first base material 16 in the form of a flat plate, a second base material 17 having an opening formed therein is placed, and in addition, a third base material 18 having an opening formed therein, which is larger than the opening in the second base material, is placed thereon. Furthermore, on the third base material 18, a seam ring 18a is disposed. Accordingly, the package base 12 has a space portion 13 formed therein so that the piezoelectric resonator element 11 can be accommodated, and in addition, the step 19 is provided for bonding the piezoelectric resonator element 11 thereto.
The mounting electrodes 14 and 14 on the surface of the step 19 are connected to external terminals 14a exposed outside the package base 12, via conduction paths 14b, passing through the layered structure formed of the laminated base materials.
Accordingly, a driving voltage applied from the external terminals 14a is applied to electrodes formed on the surface of the piezoelectric resonator element 11, via the mounting electrodes 14 and 14, and the piezoelectric resonator element 11 oscillates at a predetermined frequency.
FIG. 9 is a flowchart generally showing steps of manufacturing the piezoelectric resonator 10 described above.
In FIG. 9, first, the package base 12 is formed by using a ceramic material, such as alumina, and the mounting electrodes 14 and 14 are formed by, for example, plating so as to correspond to the piezoelectric resonator element 11.
In the step described above, since the package base 12 is formed to have a laminated structure as described above, green sheets formed of ceramic materials corresponding to individual layers are formed, respectively, and are then laminated with each other, and firing is then performed.
For example, FIG. 10 shows a green sheet corresponding to the base material 17 for the second layer shown in FIGS. 8(a) and 8(b), and shows a state in which a plurality of the base materials 17 for the second layer are formed in one green sheet which is not cut yet. On the base material 17 for the second layer, for example, as shown in FIG. 10, the mounting electrode 14 in a state connected to the conduction path 14b is formed by electroplating or the like (a conduction path 14b, which also extends to another layer, may be formed in some cases). That is, after the individual layers are laminated with each other and are then fired, the seam ring 18a is brazed thereto, and gold electroplating is performed on the external terminals 14a. In this step, as described above, since the mounting electrode 14 is connected to the external terminal 14a via the conduction path 14b, the mounting electrode 14 and the conduction path 14b are formed by plating gold (Au) on a tungsten metalized underlying layer at exposed areas thereof which have been laminated.
FIGS. 11(a) and 11(b) show the package base 12 formed of the individual base materials laminated with each other. FIG. 11(a) is a plan view of the package base 12, and FIG. 11(b) is a schematic cross-sectional view taken along plane IIb—XIb (shown in FIG. 11(a)).
As shown in FIGS. 11(a) and 11(b), when the third base martial 18 is placed on the second base material 17, the conduction paths 14b are located below the third base material 18 and are almost covered therewith, and the two mounting electrodes 14 and 14, which have been plated with gold, are exposed on the step 19.
Next, after the seam ring is brazed to the third base material 18, gold plating is performed on the exposed mounting electrodes 14 and 14 described above.
Accordingly, only on the mounting electrodes 14 and 14 which are exposed is plated with gold, and the conduction paths passing through the layers are not plated with gold.
In addition, in FIG. 9, an excitation electrode and a connection electrode are formed by deposition on the piezoelectric resonator element 11, and silicone-based conductive adhesives 15 and 15 are applied to the mounting electrodes 14 and 14 on which the piezoelectric resonator element 11 is to be mounted (ST1).
Next, on the mounting electrodes 14 and 14 of the package base 12 in FIGS. 8(a) and 8(b), the piezoelectric resonator element 11 described above is fixed by bonding using the silicone-based conductive adhesives 15 and 15 as shown in FIGS. 8(a) and 8(b) (ST2).
Next, the package base 12 is placed in a heat curing oven, which in not shown in the figures, and the silicone-based conductive adhesives 15 and 15 are dried and cured (ST3). Subsequently, when the piezoelectric resonator element 11 is sufficiently fixed on the mounting electrodes 14 and 14 by the silicone-based conductive adhesives 15 and 15 provided therebetween, a driving voltage from the external terminals 14a is applied to the piezoelectric resonator element 11, via the conduction paths 14b, and the mounting electrodes 14 and 14, and while the oscillation frequency is monitored, the weights of the electrodes are reduced by, for example, irradiating laser light on the surface of the piezoelectric resonator element 11, whereby frequency adjustment is performed (ST4).
Next, the lid body, which is not shown in the figures, is placed on the package base 12, and sealing is performed by, for example, seam welding (ST5).
As described above, the piezoelectric resonator 10 is complete.
Most of the steps described above are commonly used for a piezoelectric oscillator, which is another piezoelectric device. That is, unlike the piezoelectric resonator, since the piezoelectric oscillator has an integrated circuit mounted in the package base, accordingly, the structure and the steps thereof are slightly different from those of the piezoelectric resonator.
FIGS. 12(a) and 12(b) show an example of a conventional piezoelectric oscillator, and for the convenience of understanding, a lid body thereof is omitted in the figures. FIG. 12(a) is a plan view showing the interior of the piezoelectric oscillator by removing the lid body, and FIG. 12(b) is a schematic cross-sectional view showing the interior taken along plane XIIb—XIIb (shown in FIG. 12(a)) by removing the lid body.
In FIGS. 12(a) and 12(b) described above, the same reference numerals of the elements of the piezoelectric resonator in FIG. 10 designate elements in these figures equivalent thereto, descriptions thereof are omitted, and the different points therebetween will be mainly described.
A piezoelectric oscillator 20 has a package base 22 in the form of a box in which a space portion 23 is formed for accommodating a piezoelectric resonator element 11 in the form of a plate. One end 11a of the piezoelectric resonator element 11 is fixed on gold-plated electrodes 14 and 14, which are two mounting electrodes disposed on a step 19 formed in the space portion 23, by bonding using silicone-based conductive adhesives 15 and 15 provided therebetween, and the other end 11b is a free end.
The package base 22 is formed by laminating four base materials 26, 27, 28, and 29 formed of a ceramic, the base material 26 located at the bottom is in the form of a flat plate, and the base materials 27, 28, and 29 placed thereon are formed of ring-shaped or frame-shaped materials having the inside diameters which are increased in this order. Accordingly, the space portion 23 is formed inside the package base 22 so that the piezoelectric resonator element 11 is accommodated therein, and in addition to the step 19 for bonding the piezoelectric resonator element 11 thereto, a second step 31 is formed at an even lower place.
The mounting electrodes 14 and 14 on the step 19 are connected to an integrated circuit 21 via conduction paths 14b passing through the laminated structure formed of the laminated base materials.
In addition, on the bottom inside the package base 22, the integrated circuit 21 is mounted, and on the step 31, a plurality of electrodes 24, which are to be wire-bonded to this integrated circuit 21 by gold wires 25, is formed. Since some of the electrodes 24 and the mounting electrodes 14 and 14 are connected to each other via the conduction paths 14b, the mounting electrodes 14 and 14 will also be plated with gold.
FIGS. 13(a) and 13(b) show the arrangement of the individual electrodes by showing the package base 22 formed of the laminated individual base materials. FIG. 13(a) is a plan view of the package base 22, and FIG. 13(b) is a schematic cross-sectional view taken along plane XIIIb—XIIIb (shown in FIG. 13(a)).
As shown in FIGS. 13(a) and 13(b), on the step 19, the mounting electrodes 14 and 14 are formed, and on the step 31, the plurality of electrodes 24, which are to be wire-bonded, is formed. In addition, on the inside bottom surface, an electrode 32 for mounting the integrated circuit 21 is provided. Methods for forming these electrodes are equivalent to those described for the piezoelectric resonator 10.
Consequently, in the piezoelectric oscillator 20, when a driving voltage applied from the integrated circuit 21 is applied to the electrodes formed on the surface of the piezoelectric resonator element 11, via the mounting electrodes 14 and 14, the piezoelectric resonator element 11 oscillates at a predetermined frequency, and an output signal therefrom is inputted to the integrated circuit 21, whereby an external signal having a predetermined frequency can be obtained.
In this connection, the conventional piezoelectric device has a problem described below.
Since this problem is a basically common problem for the piezoelectric resonator 10 and the piezoelectric oscillator 20, the problem concerning the piezoelectric resonator 10 will be described.
As described in FIGS. 8(a), 8(b), 10, 11(a) and 11(b), in the conventional piezoelectric resonator 10, the mounting electrodes 14 and 14 are connected to the external terminals 14a (the electrodes 24 to be wire-bonded in the piezoelectric oscillator 20) via the conduction paths 14b. 
The external terminals 14a and the electrodes 24 are advantageously gold plated, as in the structure described above, in order to ensure solder wettability and in view of bonding characteristics or oxidation resistance, and the mounting electrodes 14 and 14 connected thereto are also simultaneously gold plated.
In addition, the electrodes of the piezoelectric resonator element 11 are bonded to the mounting electrodes 14 and 14 using the silicone-based conductive adhesives 15 and 15. Accordingly, a driving voltage applied from the outside is to be carried to the piezoelectric resonator element 11.
The silicone-based conductive adhesive 15 is used for the following reasons. That is, in the case in which the piezoelectric resonator 10 and the piezoelectric oscillator 20 are subject to conditions of varying temperature, when a conductive adhesive formed of a rigid resin, such as an epoxy-based or a polyimide-based resin, is used instead of the silicone-based conductive adhesive, and when a difference in expansion and contraction is generated between the piezoelectric resonator element 11 and the package base 12, the adhesive formed of the rigid resin described above cannot absorb the difference, and stress is applied to the piezoelectric resonator element 11, whereby degradation of characteristics, such as a change in frequency, an increase in CI (crystal impedance), or the like may occur in some cases.
In addition, in the case in which the piezoelectric resonator 10 or the piezoelectric oscillator 20 is mounted on a mounting substrate or the like, when a deforming effect caused by an external force is applied thereto and is transferred to the piezoelectric resonator element 11, via the adhesive, it is considered that the same phenomenon as described above may occur.
Accordingly, as described above, the electrodes of the piezoelectric resonator element 11 are bonded to the mounting electrodes 14 and 14 using the relatively soft silicone-based conductive adhesives 15 and 15. Since this silicone-based conductive adhesive is relatively soft, the difference in expansion and contraction and the deforming effect can be satisfactory reduced, and in addition, since a silver filler is contained in the silicone-based conductive adhesive, the electrical conductance can be obtained.
However, the silicone-based conductive adhesive 15 has a problem in that adhesive strength thereof adhered to the gold component in the mounting electrode is low.
That is, gold is an inert metal, is unlikely to be oxidized, and has an insufficient bonding force to a resin used for the adhesive. In addition to this, since the shrinkage of the silicone-based conductive adhesive is low when cured by heating (ST3 in FIG. 9), the silver filler component contained therein has a weak force penetrating into the surface of the gold on the mounting electrode 14, and hence, an electrical conductance defect may occur in some cases. Furthermore, the silicone-based conductive adhesive 15 may form a resinous layer at the interface with the gold in some cases, and as a result, the electrical conductance defect may also occur in some cases.